Printing systems that utilize the physical interaction of a laser beam with a coated film material are commercially available. The Crosfield Laser Mask system (available from the Crosfield Company of Glen Rock, N.J.), for example, utilizes a film support on which graphite particles in a binder are coated. The film support is exposed to a YAG laser. The heat generated by the absorption of the laser beam by the carbon particles causes the carbon to ablate from the film and transfer to a paper receiver. The image is built up, pixel by pixel, by removing carbon from low density areas of the image. The paper receiver constitutes a proof of the image, while the film from which the carbon was removed constitutes a negative transparency of the image. The transparency is utilized in the graphics art industry to expose or "burn" a lithographic plate.
While the system has met with some commercial success in the newspaper industry, the use of the YAG laser causes some difficulties. It is difficult, for example, to maintain and control the YAG laser, which requires substantial cooling and has a "noisy" beam in which the power varies erratically. The system also suffers from an inherent lack of resolution caused by the long wavelength of the YAG laser emission.
In order to overcome the difficulties experienced with the YAG laser, it has been suggested that a system be developed that utilizes a laser diode to expose the film support. U.S. Pat. No. 4,973,572, for example, discusses the use of a dye coating consisting of cyan, magenta and infra-red dyes in a cellulose nitrate binder which is exposed to a diode laser beam. An air stream was blown over the surface of the film support to remove sublimed dye. It has been found that the resulting dye removal gives a minimum optical density (Dmin) of 0.30. A Dmin value of 0.30, however, is too high to be generally useful in the graphic arts industry, as the piecing together of images with a Dmin of 0.30 with normal silver halide images having a Dmin of 0.04 and the exposing of a lithoplate with the composite, would result in the high Dmin image portions of the composite image formed therefrom being four times underexposed compared to the silver halide portions of the composite image. The result would be significant dot shrinkage in the underexposed portions of the image, with a corresponding change in printed density on a press. In fact, it is preferably that Dmin be limited to less than 0.11 to yield acceptable results.
The high Dmin portions of the image also suffer from visible raster lines, which have been found (as will be discussed in greater detail below) to be caused by the melting of the polyester substrate by the heating action of the diode laser beam. The melted raster lines may be viewed as a kind of non-uniformity in the image. Although the raster lines do not have an impact on contact image exposure, they do cause considerable flare in projection imaging systems like overhead projectors, and do constitute a noticeable cosmetic defect to customers accustomed to the uniform appearance of a silver halide negative.
In view of the above, it is an object of the invention to provide a method and apparatus for performing laser dye ablation printing utilizing a laser diode with improved contrast and uniformity, i.e., with Dmin reduced to preferably less than 0.11 and reductions in the appearance of raster lines.